1. Field of the Invention
The present invention relates to a three-dimensional (3D) sound reproducing apparatus, and more particularly, to a 3D sound reproducing apparatus for multiple listeners which concurrently presents the same 3D sound to multiple listeners, and a method thereof.
2. Description of the Related Art
In the audio industry, there has been efforts to reproduce sound with a full sense of presence such that an audio case is formed at a one dimensional point or on a two dimensional plane. That is, a mono system at the initial development stage, a stereo system, and recently a Dolby surround sound system are all for reproduction of sound with a sense of presence. However, as the multimedia industry develops, the aim of technologies concerning recording and reproducing aural information, i.e., a sound signal, as well as visual information, changes from a faithful reproduction with a sense of presence to a reproduction with a 3D sound space in which an audio case can be located at an arbitrary position.
Most audio apparatuses available today reproduce a stereo sound signal rather than a mono sound signal. When a stereo sound signal is reproduced, the range of a sense of presence felt through reproduced signals is limited by the installation positions of speakers. Accordingly, to improve the range of a sense of presence, studies for improving reproduction capability of a speaker and generating a virtual signal using signal processing have been conducted.
A typical system from the result of the above studies is the Dolby surround stereo system of a surround reproduction method using a set of five speakers. In this system, a virtual signal output to rear speakers is separately processed. The virtual signal is generated by delaying the signal according to spacial movement of the signal and transmitting a signal whose amplitude is reduced to the rear speakers. Currently, most home video cassette recorders and laser disk players employ such a technology called Dolby Pro Logic Surround System. Due to an apparatus employing the above technology, the quality of sound felt in the theater can be reproduced at home.
As is stated above, although sound more faithful to a sense of presence can be obtained by increasing the number of channels, as many speakers as the number of channels are required. Accordingly, costs and installation space problems occur.
These problems can be improved by applying the result of a study on how humans hear and feel sound exiting in a 3D space. Particularly, among the studies on sound recognition by humans, studies concerning both ears greatly affect recognition of a sound source in a 3D space.
The above studies on both ears concern a mutual effect of input signals input to both ears, i.e., an interaural intensity difference of the amplitude of a signal felt by the right ear and the left ear, or a difference of phase of sound input to the right and left ears generated due to an interaural time difference in transmitting sound. According to the result of research on both ears, the property of recognizing a sound source existing at one point in a space by humans has been modeled. Such recognition property is referred to as a head related transfer function (hereinafter called xe2x80x9cHRTFxe2x80x9d).
The HRTF is a filter coefficient for modeling routes from a sound source to the eardrum and characteristically has a value varying according to the relative position between a sound source and the head. The HRTF is represented as an impulse response or a transfer function at the middle ear with respect to a feature in the case in which the signal is transmitted to both ears when a sound source exists at one point in a space. By applying the HRTF, a process of transferring a position where sound exists to another arbitrary position in a 3D space can be possible.
Meanwhile, many studies have been made concerning how the hearing sense of humans can recognize a 3D sound space. A virtual sound source has recently been suggested and an actual application field is being searched for.
In general, one can best hear a stereo sound at a position that is at the apex of a regular triangle having a straight line connecting two speakers as the base thereof. However, since it is not possible to limit the position of the audience at that position, spatial problem occurs. Also, it is very difficult to adjust the balance of sound according to the position of the audience.
Aiwa, a Japanese company, has solved this problem by including a xe2x80x9cuni-orientedxe2x80x9d speaker, capable of generating a hard sound toward a listener, in a conventional speaker unit. The most characteristic of the speaker above is that the audience at any position in front of the speaker unit can enjoy a balanced stereo sound. In an ordinary speaker system, as a listener moves to the left with respect to the speaker unit, sound generated by the right speaker decreases. However, since the uni-oriented speaker included in the speaker unit is angled 45xc2x0 inwardly, the right speaker generates a hard sound to the left and a weak sound to the right. Reversely, the uni-oriented speaker of the left speaker unit generates a weak sound to the left and a hard sound to the right. Consequently, the sound generated by both the left and right speakers are balanced when the listener is positioned to the left or right.
A speaker system developed in 1993 by Japan Victor Company, another Japanese company, provides a virtual reality sound by which sound from the rear side where no speaker actually is present can be heard with only two speakers disposed at the front side. The above speaker utilizes aural hallucination by humans. Humans unconsciously search for the direction of sound using both ears. The speed of sound transferred is 340 m/sec and the distance between the ears is about 20 cm, so that the difference in time for transferring sound to both ears is 1/500 sec at its maximum. The difference in the a level of sound to both ears is also a major factor in recognizing the direction of sound. Humans recognize a source of sound by using information obtained from the two differences and the eyes. Thus, if the time for transferring sound to both ears can be controlled, sound generated from only two speakers can cover the whole room so that listener can feel as if he/she were sitting in a theater.
However, all 3D sound-related technologies having been developed so far targets a single listener. That is, the current audio reproduction system provides an effect of stereo sound when a single listener is positioned at the apex of a regular triangle using a straight line between two speakers as a base. Thus, in the case of multiple listeners, the same and concurrent stereo effect is not possible.
Such a problem becomes serious in a case of a home theater system. As shown in FIG. 1, when all family members are seated around a sound source, a conventional home theater system cannot provide all family members with good stereo sound.
Recently, there are suggestions to provide a sense of presence and space using more speakers by providing a Dolby Pro Logic system instead of the two channel reproduction. However, in the above system, a plurality of listeners should be positioned at the center of a circle connecting each speaker to enjoy a complete 3D effect. Further, in order to reproduce multi-channel audio, corresponding multiple speakers and an amplifier to drive each speaker should be provided. Thus, problems of costs and installation space occur.
To solve the above problems, it is an objective of the present invention to provide a 3D sound reproducing apparatus which provides with multiple listeners, regardless of their positions, the same 3D sound effect at the same time, and a method thereof.
Accordingly, to achieve the above objective, there is provided a 3D sound reproducing apparatus for multiple listeners which includes an inverse filter module for filtering an input sound signal such that each of the listeners can have the same virtual sound source, time multiplexing module for sequentially selecting one of the sound signals filtered by said inverse filter module at a predetermined interval, and a plurality of speakers for outputting the sound signal selected by said time multiplexing means as sound.
According to another aspect of the present invention, there is provided a method for reproducing an input sound signal through a fixed number of two or more speakers to provide the same 3D sound effect to multiple listeners, which includes the steps of obtaining a speaker transfer function which models a route between said speakers and an ear of each of the listeners, (b) obtaining filter values by multiplying the inverse matrix of the speaker transfer functions by a virtual sound source transfer function which models a route between a virtual sound source and an ear of a listener, (c) sequentially selecting one of the filter values in order at a predetermined interval; and (d) convolution-processing an input sound signal with the selected filter value and outputting the result of the convolution process to the speaker.